As LED lighting technology improves, cost-effective and energy-efficient LED lighting will become dominant in high power lighting applications due to its high luminous flux, high efficacy, and long life span. AC-connected high-power LED lighting is commonly used in outdoor lighting applications such as street lights, parking lot lights and high bay industrial lights. In high power applications, the power supplies (i.e., LED drivers) typically operate at Vin=85 to 265 Vac and PO>50 W. In these applications, many LEDs are connected in series to increase the light output. This configuration results in a combination of a high output voltage and low output current requirement for the drivers, and usually requires isolation.
Conventional approaches for high-power LED drivers include two categories: (a) two-stage configurations and (b) single-stage configurations, as shown in FIGS. 1A and 1B. The two-stage configuration shown in FIG. 1A can achieve high power factor and tight output current regulation, making it a mainstream solution for high-power LED drivers. However, it exhibits poor efficiency and low power density. Large capacitors (generally around 1 μF per Watt) are typically required at the output of the power factor correction (PFC) stage to buffer the power difference of the AC input and DC output. The PFC output capacitance can be reduced by increasing the voltage ripple. However, capacitance reduction is limited because the PFC output voltage must be higher than the minimum input line voltage to ensure normal operation of the PFC converter. Therefore, electrolytic capacitors are still required for two-stage high power applications.
Although the conventional single-stage configuration, shown in FIG. 1B, presents a cost-effective and high-efficiency solution, large electrolytic capacitors are still necessary to reduce the double line frequency (e.g., 100 Hz or 120 Hz) current ripple. Otherwise, the double line frequency current ripple will cause light flickering, which is harmful to human visual system.
Electrolytic capacitors suffer from relatively short operating life (e.g., 5,000 hours), compared to the lifetime of an LED lamp (e.g., >50,000 hours). Also, electrolytic capacitor life span is a function of operating temperature, as the life span halves for each 10° C. increase in operating temperature. Therefore, the life span problem becomes more significant in outdoor applications such as street lighting. Clearly, the use of electrolytic capacitors in these conventional approaches presents a significant drawback.
Solutions have been proposed to reduce the ripple and remove the electrolytic capacitors. For example, one method is to inject the harmonics (3rd and 5th) into the input current. However, this sacrifices the power factor, which is unacceptable for high-power LED drivers. In another approach, an auxiliary ripple cancellation stage is connected in parallel with the PFC output to reduce the PFC output capacitance, as shown in FIG. 1C. In this approach the auxiliary stage must withstand a high voltage stress equal to the PFC output voltage. The voltage (Vaux) across the auxiliary capacitor (Caux) is even higher. Therefore, high voltage-rating components are necessary, which generally have higher cost and lower efficiency than their low-voltage counterparts. In addition, an output filter inductor LO is required to be connected in series with the LED load to limit the LED current ripple.